http://www.gnucitizen.org/blog/google-gmail-e-mail-hijack-technique/

I read here that there is a hack where a malicious site can get into your gmail account, install a filter (such as any email that has an attachment or is from a bank etc.) and forward it to another email account. The filter is installed while you are logged into your gmail (or google homepage or any other google service) and then when visiting the site, the filter is installed in the background. The worst part is that email hijacker would continue getting your emails as long as the filter is in place, not as long as you visit their malicious website.

To secure oneself one should check their gmail filter for unwanted insertions and log out of gmail while surfing unknown sites.